Oxide superconductors hold promise for achieving significant improvements in the efficiency, size, and weight of a wide variety of electrical equipment, such as motors, generators, transformers, magnets, and transmission lines. For many of these applications considerable lengths of conductor must be fabricated that have critical current densities of at least 10.sup.4 -10.sup.5 A/cm.sup.2 in magnetic fields of one tesla (T) or greater at operation temperature. The need to produce large quantities of conductors economically has stimulated effort worldwide to develop processes for fabricating polycrystalline wires and tapes from oxide superconductors belonging to the Y-Ba-Cu-oxide (YBCO), Bi-Ca-Sr-Cu-oxide (BCSCO), and Tl-Ca-Ba-Cu-oxide (TCBCO) families. Key to the success of this work is the ability to prepare polycrystalline conductors which have both excellent intergranular connectivity and excellent magnetic flux pinning characteristics.
Methods of preparing thallium system superconductors are documented in the literature. One method of preparation is reported in "Bulk Superconductivity at 120 K in the Tl-Ca-Ba-Cu-O System," Z.Z. Sheng and A.M. Hermann, Nature Vol. 332, Mar. 10, 1988, pp. 138-139. Briefly described, appropriate amounts of powdered Tl203, CaO, and BaCu.sub.3 O.sub.4 were mixed, ground, and pressed into pellets. The pellets were heated in a furnace at 880.degree.-910.degree. C. with an oxygen flowing atmosphere for three to five minutes. After heating, the pellets were quenched to room temperature in air or furnace cooled to room temperature. Basically, the pellets were reaction sintered by the heat treatment, forming a superconducting conductor.
Thin films of thallium superconductors have also been made by sequential thermal evaporation, sequential electron beam evaporation, and spray pyrolysis of nitrate solutions containing the precursor cations, such as calcium, copper, and barium. A limitation for these methods is the time needed to deposit a layer about 10-100 gm of the precursor deposit on the substrate.
The above-mentioned sintered ceramic pellets, thin films, and spray pyrolysis nitrate solutions are subsequently treated by heating in flowing air or oxygen at temperatures of about 800-900.degree. C. to form the superconducting compositions. A drawback for these systems is that thallium oxide has an appreciable vapor pressure at the temperature required to form the superconductor. As a result, thallium can be vaporized during the 800-900.degree. C. annealing temperatures leading to the loss of thallium from the superconductor.
A further drawback to the above methods of making thallium superconductors is that low current carrying capacity occurs when the grains are weakly linked or poorly connected. Generally, the superconducting films have a non-textured microstructure with poor critical current. Often the grains have the c-axis parallel to the substrate surface.
To achieve high current carrying capacity in thallium superconductors, the microstructure of the superconductor needs to have strongly aligned crystal microstructure with the c-axis perpendicular to the plane of the substrate. In the past, methods and apparatuses have not disclosed the means of making continuous long lengths of thallium superconductors that achieve the desired microstructure for high current carrying capacity.
Recently, a method for making high critical current and high critical current density thallium superconductors having the c-axis perpendicular to the plane of the substrate has been disclosed in commonly owned and assigned U.S. patent application, Ser. No. 08/537,578, incorporated herein by reference.
Thus, there is a need for an apparatus to fabricate continuously long lengths of thallium-containing superconductors that produces a uniform superconducting phase and microstructure over long lengths of superconductor material with the c-axis aligned perpendicular to the plane of the substrate. There is an additional need for an apparatus to fabricate long lengths of superconducting conductors rapidly and economically by continuous and batch processes. The long lengths of thallium superconductor can be used for magnet and transmission applications, to mention a few.